Steve Schimmrich is a geologist and community college professor in a rural area of the mid-Hudson Valley of New York. All of the opinions expressed here are strictly his own. Sometimes he gets cranky and uses bad words.

That circled pebble is a bit over a centimeter across. This outcrop represents a sedimentary rock known as conglomerate - pebbles and sand cemented together by silica (typically). What do geologists think when they see rounded pebble conglomerates like this? Running water. Streams. These pebbles are too large to have been transported by winds and the rounding indicates water transport (look at the sediments in the bottom of any stream to see similar pebbles).

Here on Earth, these types of rocks are very common.

Above is an example of a local conglomerate from the Shawangunk Ridge on the Mohonk Preserve. It formed from braided streams flowing off the ancient Taconic Mountains to the east over 420 million years ago.

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Above is a slightly different example of a local conglomerate. This is from the Catskills near North Lake and the results of rivers flowing off the ancient Acadian Mountains a bit less than 400 million years ago.

Not much different from the stuff we now see on Mars!

So, back to Mars. Why a river deposit here? Because we're near the outlet of a 30 meter deep canyon (Peace Vallis) that cuts through Gale Crater. At the mouth of the canyon is a feature called an alluvial fan. Alluvial fans form when water flows fast through a canyon and then comes out onto a valley floor. As the water spreads out, it slows down. As it slows down, it drops sediment. A fan-shaped wedge of sediment then develops. Here's an alluvial fan on Earth (for scale, that's a road cutting across it).

Here's the alluvial fan on Mars. The oval was the targeted landing area for the rover and the plus sign is where it actually touched down.

The Hottah outcrop represents a stream channel coming off of that alluvial fan further confirming what we already know. Liquid water once flowed over the surface of Mars!